1. Field of the Invention
This invention relates to digital transmission system modems based on a carrier wave phase modulation with 2, 4 or 8 phase states. Nevertheless, this invention concerns also a carrier wave amplitude modulation with several amplitude states.
2. Description of the Prior Art
More particularly, the invention concerns an arrangement included in a modem for recovering carrier wave and clock frequencies in a digital transmission system. In a transmitting part are provided a binary signal associated with a clock signal and also a carrier wave having a frequency P given in a predetermined ratio n with the clock signal frequency 1/T. The carrier wave is phase or amplitude modulated by the binary signal into a modulation signal defined by I=2.sup.J phase or amplitude states corresponding to combinations of J successive bits in the modulating binary signal. In a receiving part is provided a phase locked loop designed to recover the frequency P of the carrier wave in the modulation signal with a view to phase or amplitude demodulating the modulation signal into the binary signal and to recovering the clock frequency.
In known transmission systems carrying out a phase-state modulation and a coherent demodulation, a carrier wave is generated by a local oscillator in a transmitting part of the system. The carrier wave has a phase undetermined with respect to clock signal provided with a binary information signal to be transmitted. The majority of carrier wave recovery means in known system receiving parts comprise a circuit for multiplying the modulation signal frequency by I so as to suppress the phase modulation and recover a signal having a pure frequency IP. A phase locked loop is usually designed to cue a voltage controlled oscillator to the frequency IP and to recover a carrier wave synchronous with the transmitting part carrier wave after dividing the pure frequency by I at an output from the phase locked loop. However, the phase of the recovered carrier wave is not determined with respect to the clock signal for subsequent demodulation of the received signal. Indeed, the phase of the carrier wave recovered at the output from the frequency divider is referred spontaneously on one of the I phases defined by the modulation.
To resolve any phase ambiguity in the receiving part and hence retrieve J-bit sequences of the binary signal respectively in terms of the corresponding phase states, use is made of a transcoding operation on J binary streams corresponding to the like-rank bits in the J-bit sequences of the binary signal. The transcoding operation can consist of a DPSK modulation or a special coding such as a delay in one of the binary streams or an insertion of a specific binary word, so as to resolve phase ambiguity in a transcoder in the receiving part. The complexity of the transcoder increases in terms of the number I of phase states and affects at least half of the cost of the modem. Transcoding also multiplies the number of errors.